Compact precision measuring head, which is resistant to high-pressure, for measuring the optical refractive index in liquids

ABSTRACT

A high-pressure resistant compact precision measurement head for highly exact optical refractive index measurements in liquids and/or gases comprising a refractive index reference body, mounted on a high-pressure bulkhead fitting. The body is composed of a single, optically homogenous material produced by pre-calculation of its geometry economically, mechanically and automatically with extremely low tolerances. The measurement head can be formed in such a manner that it has on its front portion an especially small measurement volume to as little as less than 0.5 mm 3 , through which a very thin optical measurement ray passes. The latter is produced behind a high-pressure bulkhead fitting in a pressure-protected interior cavity of the instrument and also evaluated there as an incoming ray after passing through the measurement medium. The precision measurement head possesses thereby no assembly and adjustment elements of optical components in the high-pressure area of the measurement medium.

BACKGROUND OF THE INVENTION

1. Field of the Invention

For a long time, refractometers have been widely used to determine theoptical refractive index in liquids and gases in which in the mostdiverse forms of the device the exit angle of the refracted light ray inits transition from the measurement medium to a reference medium isdetermined quantitatively. The basis of these measuring instruments isSnellius' Law of Optical Refraction. The achievable accuracy of therefractive index of the medium being studied is thus dependent, amongother factors, on the accuracy of the optical refractive index of thereference body and of the angles α and β of the incident and therefracted light ray, as well as on certain characteristics of the lightsource and of the detector measuring the incoming ray. In order toachieve maximum accuracy in measurements it is necessary to fulfill thehighest stability requirements, especially in some optical/mechanicalcomponents and their alignment in the optical bank of the measurementinstrument. The absolute values of the angles, the measurements and therefractive index of the reference body need not necessarily be knownvery exactly, since the refractometer can be calibrated using one ormore liquids and/or gases the precise optical refractive index of whichis/are exactly known.

2. Description of the Related Art

Refractometers of especially high accuracy in in situ measurements inthe ocean, as well as in the laboratory, have a significant role indetermining the physical state quantities of ocean water, especially inthe extensive spaces of the deep sea. Thus for a long time attempts havebeen made to create appropriate instruments for field use, but thestability and/or accuracy achieved has remained somewhat unsatisfactory.Nonetheless, refractive index accuracy in the range of 10⁻⁶, possiblyeven to 10⁻⁷, is required for meaningful refractive index measurement inthe ocean, which means that in practice refractive angle measurementswhich are stable and maximally long-term constant in the magnitude ofone-tenth arc second must be achieved at hydrostatic environmentalpressures up to ca. 1000 bar.

To date it could be shown that such high stability requirements arebasically achievable, as for example evident in recently described andexperimentally tested field instruments. (OCEANS '88, IEEE Publ. No.88-CH 2585–8, Baltimore, Md., USA, Volume 2(4), 497 . . . 504, (1988);OCEANS '99, MTS/IEEE Publication, Seattle, Wash., USA, ISBN:0-933957-24-6, Vol. 3, 1218 . . . 1222, (1999)).

BRIEF SUMMARY OF THE INVENTION

The invention concerns a high-pressure resistant compact precisionmeasurement head for highly exact optical refractive index measurementsin liquids and/or gases comprising a refractive index reference body,mounted on a high-pressure bulkhead fitting. The body is composed of asingle, optically homogenous material produced by pre-calculation of itsgeometry economically, mechanically and automatically with extremely lowtolerances. The measurement head can be formed in such a manner that ithas on its front portion an especially small measurement volume to aslittle as less than 0.5 mm³, through which a very thin opticalmeasurement ray passes. The latter is produced behind a high-pressurebulkhead fitting in a pressure-protected interior cavity of theinstrument and also evaluated there as an incoming ray after passingthrough the measurement medium. The precision measurement head possessesthereby no assembly and adjustment elements of optical components in thehigh-pressure area of the measurement medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the overall principal of the refractometer using the wellknown sensor head configuration with several separated mechanical andoptical components A mounted and adjusted on the bulk head B.

FIG. 2 shows the basic refraction prism with fundamental ray path of theincident beam at nearly grazing angle.

FIG. 3. illustrates how unnecessary areas of so far known referenceprisms are eliminated in order to improve hydrodynamic rinsing of themeasurement area of the sensor.

FIG. 4. shows the main section through a typical single block sensorhead and the major light path.

FIG. 5. shows a reference prism of the sensor head comprised of acircular cylinder with fastening groove a, and polished upper and lowercylinder areas b and c for high pressure O-ring type sealings.

FIG. 6. is a side view of the prism in FIG. 5.

FIG. 7. is a vertical view of the prism from above.

FIG. 8. is a spatial sketch of the upper part b of FIG. 5; e marks thearea where the measuring light hits at a nearly grazing angle.

FIG. 9. is a spatial sketch of the upper part b of FIG. 5; and shows thepath of a ray, as well as surfaces which may be plated.

FIG. 10. shows a high-pressure bulkhead fitting. The refraction indexreference is provided with a polished flat surface in its lower part torest as a high-pressure resistant, planar, undislodgable base on theplanar polished surface of the fitting. High pressure O-ring sealingscan be done at j and k.

FIG. 11. shows the entire unit of the high-pressure resistant compactsingle block precision measuring head for highly exact opticalrefractive index measurements in gases and liquids.

In order to be able to classify the inventive object exactly, the basicrefractometer principle will be described briefly here, with theassistance of FIG. 1. At first glance this is an instrument which in itsexterior dimensions such as length and diameter is comparable to thewell-known, classical Abbé submersible refractometer. The rod-shapedmeasuring device has in its one end the sensor measuring head A to besubmerged in the liquid (or gas) to be tested, and a housing Ccontaining the optical bank, light source and sensor electronics. Themeasuring head and the housing wall are separated by a high-pressurebulkhead fitting B, through which especially the optical measurementsignals can be conducted in a highly stable manner without, for example,experiencing an uncontrolled influence of the measurement value and thusan unacceptable reduction of measurement accuracy due to compression.For all measurements the device can be completely submerged in themedium. The necessary supply and signal connections from the sensorinterior to the outside and vice versa can, for example, be achieved bymeans of pressure resistant plugs or cable fittings or also, forexample, via magnetic or optical transmission paths through the housingwall to the extent the housing material has no magnetic or opticalshortcomings. Pressure resistant glass or ceramic housings are examples.

DESCRIPTION OF THE INVENTION

For a description of the measurement process refer to FIGS. 1, 2 and 3.FIG. 1 shows the functional principle of the refractometer inschematically represented blocks, FIG. 2 the path of the rays withreference to the refractive index prism in which ε indicates the prismangle between the light entry plane at the boundary between themeasurement medium and reference prism and the plane of the seat of theprism on the high-pressure bulkhead fitting. FIG. 3 illustrates how, forexample, unnecessary areas of the reference prism can be eliminated bymeans of grinding in order to improve hydrodynamic rinsing of themeasurement area of the sensor without influencing the active measuringsurfaces and without compromising the pressure resistance. Well-definedlight emanating from light source 1 is divided, for example, in anoptical fiber coupler 2. Part of the light is passed via a lightwaveguide through the high-pressure bulkhead fitting B into thecollimator 3 where it is bundled to a fine collimated light beam. Afterpassing through the measurement medium it strikes the refractive indexreference prism 4 under the angle of incidence α, is refracted in thedirection of the perpendicular under the angle β, finally passes againthrough the high-pressure bulkhead fitting into the pressure-protectedinner housing and reaches the photoelectric receiver 5, which permitsextremely exact determination of the refractive angle. The variousphotoelectric currents of the light converter are converted with the aidof the transimpedance amplifier 6 and 7 into highly precise electricvoltages. From these voltages it is possible, in the known manneraccording to the state of the art, to calculate the refractive index ofthe tested medium electronically and fully automatically to ca. 10⁻⁷after the coefficients of the conversion algorithm have been determinedin a previous calibration process. For exact monitoring of the lightsources used, or their light wavelengths, another part of the opticalwave fork coupler 2 is led into the light wavelength measurement module10, which supplies a calibrated electrical voltage U_(λ), from which thewavelength can be explicitly calculated and then in appropriate mannerused for the mathematical determination of the refractive index. In thesame manner, fine correction of the residual influences of pressure andtemperature on the measurement head can be made by separate measurementof these magnitudes on the reference prism on the high-pressure bulkheadfitting. For this purpose there is a thermometer 8 mounted on themeasurement head, which makes available the calibrated measurementvoltage U_(T) for subsequent data processing. Ambient pressure isdetermined by means of a separate pressure gauge.

According to this basic principle, absolute refractive index accuracybetween 10⁻⁶ and 10⁻⁷ in situ is routinely possible if there is acorresponding high-pressure resistant, long-term stable precisionmeasurement head with a reference prism. The versions of the sensor headrealized to date, which have been tested in the ocean and in thelaboratory, were comprised of numerous costly individual parts whichposed significant and difficult to fulfill demands with regard topressure resistance, stability and accuracy, which were moreover verycostly to assemble, adjust and align, and for which costly, customadjustment devices and measurement instruments were required, so that onthe whole the previous measuring heads were very critical in theirmanufacture and subsequent maintenance of accuracy. In addition,production costs were sufficiently high that the utilization of largenumbers of them in networks in the ocean in so-called expendable,one-way probes did not appear at all feasible.

The invention described here has to do with a completely new solution ofthe sensor head problem by forming the critical sensor part from asingle homogenous optical material which can be manufactured completelyin automatic machine production and contains no foreign parts requiringadjustment. The typical form of this so-called “single block” sensorhead is in principle shown in FIG. 4: the view shows a main sectionthrough the reference prism body 1 and high-pressure bulkhead fitting 2,which are firmly connected with each other and pressure-sealed. The finecollimated light beam (for example 0.5 mm diameter), which comes out ofthe pressure-protected interior cavity of the instrument through aboring in the bulkhead fitting, is reflected on the mirroring surface 3in such a manner that it exits surface 4 vertically into the measurementmedium and after a short distance strikes the entrance surface 5, whereit is refracted into the reference prism to the perpendicular 6. On theadditional mirroring surface 7 the beam is then reflected again into theinterior of the instrument through the corresponding boring in thehigh-pressure bulkhead fitting and there finally reaches the detectormeasuring the refractive angle. Individually it is possible to mirrorcoat the surfaces 3 and/or 7 or to achieve the reflection by means oftotal reflection on the corresponding surfaces.

The part of the sensor head thus comprised of a single glass materialand possibly having, for example, applied mirror coatings of gold on theindicated optical parts, can be manufactured completely mechanically bysawing, grinding, polishing and possibly mirror coating following priormathematical determination of the collective angles, surfaces andmeasurements, and according to the selection of the appropriate opticalglass material and determination of the measurement range appropriatefor the liquids or gases to be measured. Regardless of whether totalreflectivity on the relevant mirror surfaces is achieved or not, goldfacings can, for example, be applied for additional protection of thesurface with regard to even the smallest mechanical or chemicalblemishes, since given the extremely high measurement precision even thesmallest imperfections can lead to a significant displacement of thefocal intensity of the light ray, which then would be a cause formeasurement deviations.

In very many applications, especially in oceanography, streaming is fromthe sensor head side toward the sensor in the direction of theinstrument axis. To optimize the streaming behavior, especially for thepurpose of faster, particularly more effective free rinsing of themeasurement volume in the light ray between the surfaces 4 and 5 in FIG.4, the shadow-free slant position shown here of these two surfaces isselected and free grindings are simultaneously undertaken to remove asmuch of the prism material as possible so that optimal streamingbehavior and the fastest possible free rinsing in the area of themeasurement volume are achieved, without however compromising therequired mechanical stability of the measurement head and withoutdisturbing the measurement ray in the interior of the glass body or inthe area of the measurement volume of the medium to be tested.

FIG. 5 shows the reference prism of the sensor head comprised of acircular cylinder with the ground fastening groove a, which allows theprism to arrange with reference to the upper cylinder area b and thelower cylinder area c. The latter facilitates problem-free O-ringhigh-pressure sealing of the glass body on the high-pressure bulkheadfitting part 2 of FIG. 4, described below. The upper cylinder part b canbe used as sealing surface for a protective cover turned back onto thesensor or for a capsule-like container with preservative and/orcalibration liquid.

FIG. 6 is a side view of the prism in FIG. 5 seen turned 90 degrees fromthe left, while FIG. 7 is a vertical view from above. In theseillustrations it is shown, for example, how hydrodynamic imperfectionscan be ground away, whereby an optimum between streaming-favorable formand mechanical stability can be achieved. The especially critical area din FIG. 5, directly at the place where the optical fiber enters themeasurement medium, must be very exactly controlled, for example madewith a narrow saw blade as a simple groove cut, in order to form in asimple manner the less critical sloping surfaces around the ellipticalarea of the optically used refractive surface e (see FIG. 8).

FIGS. 8 and 9 are spatial sketches of the upper part b of FIG. 5 for abetter visualization of the streaming-optimized design of the “singleblock” sensor head. FIG. 9 shows especially the path of the ray as wellas any surfaces which may be plated e.g., with gold Au.

The refractive index reference body is provided with a polished flatsurface in its lower part c according to FIG. 5, which rests as ahigh-pressure resistant, planar, undislodgable base on the planarsurface f of the high-pressure bulkhead fitting according to FIG. 10.Spring tabs g press downward on the glass body in the direction of theinstrument axis. A precisely-fitting centering ring h protects againstradial displacement. The clamping ring i is, for example, firmlyconnected by threading to the main body 1 of the high-pressure bulkheadfitting and has O-ring seals k and j, which are durable with referenceto the medium to be measured.

The entire unit of the high-pressure resistant compact precisionmeasuring head for highly exact optical refractive index measurements ineither at rest or flowing liquids and for gases is shown in FIG. 11. Aversion has been produced according to FIG. 11 with a glass cylinderdiameter of 27 mm. Function and high-pressure resistance weredemonstrated.

Finally, it is to be noted that according to the type of application thespace between the surfaces 4 and 5 in FIG. 4 can be variously formed.There is a given constant measurement volume with constant ellipticalcross-section of the entry ray through surface 5 into the measurementprism, especially in the case of the vertical exit of the measurementray through surface 4 into the measurement medium. In this case theangle between the surfaces 4 and 5 is smaller than 90 degrees.

One can however also increase the angle between the surfaces 4 and 5 tomore than 90 degrees in order, for example, to increase the measurementsensitivity, i.e. to increase the change of the entire deflection anglewith the change of the refractive index in the fluid, in order then tolet the measurement ray pass through both surfaces obliquely, whichhowever somewhat changes the measurement volume according to locationand form and especially also the elliptical cross section of the lightray as it passes through surface 5 according to position and form, ifthe refractive index of the measurement medium changes. This means thata somewhat differently situated area of the measurement prism isutilized.

For the production of the homogenous glass prism it is to be noted thatit can be made from a single work piece but also can be put togetherfrom several segments, which then for example are diffusion welded, sothat optical and also mechanical homogeneity are completely assured.

1. A high-pressure resistant compact precision measurement head forhighly exact optical refractive index measurements in liquids and/orgases comprising a refractive index reference prism body cut out of onesingle circular piece of reference material, mounted on a high-pressurebulkhead fitting, which bulkhead fitting is attachable to a housing of ameasurement instrument, which measurement instrument contains a lightsource for directing a measurement light ray into the reference prismbody, which body is composed of optically homogenous material andwherein the reference body has space defining measurement surfaces, onits front portion for defining a probe volume of the medium to bemeasured of less than 0.5 mm³, through which measurement medium ameasurement light ray can pass, the measurement light ray beingproducible in a pressure-protected interior cavity of the measurementinstrument and evaluated as an angular deflection of the exiting lightray after passing through the measurement medium.
 2. The high-pressureresistant compact precision measurement head according to claim 1 whichis arranged such that the measurement light ray exiting the measurementmedium exits either vertically from the reference body into the mediumto be measured, so that a refraction occurs only at one measurementsurface of the space in the reference body, or the measurement light rayenters at oblique angles into the measurement medium in which case themeasurement beam is refracted at two surfaces.
 3. The high-pressureresistant compact precision measurement head according to claim 1wherein its refractive index reference body has sensing surfaces of lessthan 1 mm² which are planar, polished and so positioned that there is noshadowing while streaming the measurement medium from forward along themeasurement instrument.
 4. The high-pressure resistant compact precisionmeasurement head according to claim 1 wherein the measurement surfaceshave edges with sloping bevels.
 5. The high-pressure resistant compactprecision measurement head according to claim 1 wherein the refractiveindex reference body has a cylindrical surface for high-pressure sealingand a planar surface as a base for attaching on the high-pressurebulkhead fitting.
 6. The high-pressure resistant compact precisionmeasurement head according to claim 5 wherein the refractive indexreference body has a groove on its cylindrical surface, through whichthe attaching on the high-pressure bulkhead fitting can be achieved withpressure claws.
 7. The high-pressure resistant compact precisionmeasurement head according to claim 5 wherein the refractive indexreference body has an additional cylinder surface above a fasteninggroove for sealing a protective cap or an ampoule-like container forpreservation liquid or gas or a standard refraction index liquid or gas.8. The high-pressure resistant compact precision measurement headaccording to claim 1 wherein the refractive index reference body hasdeflection surfaces which are either totally reflective or are platedwith a mirror coating for deflection of the measurement light ray priorto entry into the measurement medium and/or after re-entry into therefractive index reference body.
 9. A device for optical refractiveindex measurement with a precision of up to 10⁻⁷ in flowing or stillliquids or gases at pressures up to 10000/bar, comprising ahigh-pressure resistant, compact sensor measurement head comprising arefractive index reference body and a measurement instrument housingseparated by a high-pressure bulkhead fitting, the measurementinstrument containing an optical bank, a light source and a sensor forreceiving an exiting light ray from the reference body, wherein therefractive index reference body which is mounted on the high-pressurebulkhead fitting comprises a single optical, monolithic material blockwherein the measurement head is so formed that it has a measurementmedium in its front part with a volume smaller than 0.5 mm³ for a finecollimated light beam directed therethrough.